In September 1977, the world's first commercially available local area network was first put into service at Chase Manhattan Bank, New York, as a beta-site.

Short for Attached Resource Computer network, ARCnet was developed by Datapoint Corporation in San Antonio, Texas.
It was defined as a group of nodes that communicates to one another over a geographically-limited area usually within one building or a campus of buildings.

It was the simplest, and least expensive type of local-area network. It used a token-ring architecture, supported data rates of 2.5 Mbps, and connected up to 255 computers. A special advantage of ARCnet is that it permitted various types of transmission media to be mixed on the same network: twisted-pair wire (Max. extension 150 meters), coaxial cable (650 m.) and fibre optic cable (2.000 m.)

Today, ARCnet is still alive in numerous companies all over the world and has more than seven million installed nodes.

Comment from Chris Piggott:
This is not so much a correction but more of a comment from the users point of view.

I worked for Chase Manhattan Bank from 1976 until 1986 and was one of the first to use the ARC in live commercial environment. I remember Gordon Peterson, Johnathan Schmidt and others at Datapoint well. ARC allowed Datapoint to 'punch far above its weight' when compared to the IBM, DEC & Wang alternatives at the time. Not bad considering its eight bit architecture. The first system we developed using ARC was a funds transfer system connected to the New York Clearing House, processing thousands of transactions per day, worth Tens of Billions dollars. At its peak, Chase had scores of individual networks installed, some with 20 or more linked processors.

Such a pity that Datapoint didn't licence the ARC technology, rather than keeping it as a proprietory product.

One additional piece of information. Chase used Infrared transmission equipment, supplied by Datapoint, to link its ARCnet between its buildings in lower Manhattan in addition to the other methods mentioned.